AW: AW: AW: [PATCH 07/12] kern_tc.c: Remove verification of th_generation for pps functions

[Sebastian Huber]

On 27.04.22 15:13, Gabriel.Moyano at dlr.de wrote:
>> On 07/04/2022 15:16, Sebastian Huber wrote:
>>> On 07/04/2022 15:10, Gabriel.Moyano at dlr.de wrote:
>>>>> Which sequence of function calls and timings cases the problem? This
>>>>> should be definitely a test case.
>>>> The generation is updated every time tc_windup() is called. So it is
>>>> more or less a race condition when it happens.
>>>
>>> This is not a race condition. The sequence counter is supposed to
>>> ensure a consistent state. You can't simply remove this.
>>>
>>> Maybe the PPS support requires that we use two timehands even for the
>>> uniprocessor configuration. For this, we have to understand under
>>> which conditions and use cases the generation change is an issue.
>>
>> In kern_tc.c there is this comment, which is a result from a discussion on a FreeBSD mailing list:
>>
>> /*
>>    * In FreeBSD, the timehands count is a tuning option from two to 16.  The
>>    * tuning option was added since it is inexpensive and some FreeBSD users asked
>>    * for it to play around.  The default value is two.  One system which did not
>>    * work with two timehands was a system with one processor and a specific PPS
>>    * device.
>>    *
>>    * For RTEMS, in uniprocessor configurations, just use one timehand since the
>>    * update is done with interrupt disabled.
>>    *
>>    * In SMP configurations, use a fixed set of two timehands until someone
>>    * reports an issue.
>>    */
>>
>> We definitely need a test case for this corner case. Adding a second timehand has a size impact. On a 32-bit machine there is
>> sizeof(struct
>> timehands) == 120.
>>
> What do you think about something in the middle between adding a second timehand and not adding it at all for non-SMP configurations?
> Let me explain better the idea; the pps_event() uses only 3 variables of the object timehand: th_offsetcount, th_bintime and th_scale. The values of them could be saved instruct  pps_state during pps_capture(). This will only happen in case of non-SMP configurations. Then, the verifications need to be adapted for SMP or for non-SMP, e.g. the th_generation will be checked at the end of pps_capture() for both configurations but only for SMP in pps_event().

I would keep the synchronization as it is in FreeBSD for now and add a 
test case which fails in non-SMP configurations. This can be used to 
discuss the trade-offs. We need such a test case anyway.

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